Combustion product detector and method of calibrating

ABSTRACT

A combustion product detector of the ionization chamber type comprises a reference ionization chamber and a sampling or signal chamber connected in series across DC lines supplied from a regulator with an insulated gate field effect transistor connected in parallel therewith and having its gate connected to the common junction of the chambers. An adjustable reference voltage is supplied to a comparator which compares that voltage with the output signal from the field effect transistor. An alarm signal is generated when the difference in voltages approaches zero and is employed to actuate the alarm or indicator. The circuit is reset automatically upon a drop of predetermined voltage at the regulator input. A voltmeter may be employed for calibration purposes - the voltage difference at the comparator during the quiescent state of the system indicates the density of combustion product at which the system is set to activate the alarm.

United States Patent [1 1 Ried, Jr.

[451 Feb. 11, 1975 COMBUSTION PRODUCT DETECTOR AND METHOD OF CALIBRATING [75] lnventor: Louis Ried, Jr., Boulder, C010.

[73] Assignee: Fire-Alert Company, Wheat Ridge,

[22] Filed: May 7, 1973 [21] Appl. No.: 358,041

[52] US. Cl 340/237 S, 250/381 [51] Int. Cl. G08b 17/10 [58] Field of Search 340/237 S; 250/381, 382, 250/384, 385

[56] References Cited UNITED STATES PATENTS 3,500,368 3/1970 Abe 340/237 S 3,530,450 9/1970 Walthard et al. 340/237 S UX 3,559,196 l/l971 Scheidweiler 340/237 S 3,564,524 2/1971 Walthard et a1. 340/237 S UX 3,573,777 4/1971 Kompelien 340/237 S 3,676,680 7/1972 Scheidweiler et al. 340/237 S X 3,735,375 5/1973 Blackwell 340/237 S Primary Examiner.lohn W. Caldwell Assistant Examiner-Daniel Myer Attorney, Agent, or Firm--Wm. Griffith Edwards [57] ABSTRACT A combustion product detector of the ionization chamber type comprises a reference ionization chamber and a sampling or signal chamber connected in series across DC lines supplied from a regulator with an insulated gate field effect transistor connected in parallel therewith and having its gate connected to the common junction of the chambers. An adjustable reference voltage is supplied to a comparator which compares that voltage with the output signal from the field effect transistor. An alarm signal is generated when the difference in voltages approaches zero and is employed to actuate the alarm or indicator. The circuit is reset automatically upon a drop of predetermined voltage at the regulator input. A voltmeter may be employed for calibration purposes the voltage difference at the comparator during the quiescent state of the system indicates the density of combustion product at which the system is set to activate the alarm.

8 Claims, 4 Drawing Figures VOLTAGE TRJENTEUFEBWBTS j 3,866,195

SHEETIGFZ VOLTAGE M REGULATOR w l 27 49 TEST TERMINAL P/"JENTED F551 1 75 SHEET 2 OF 2 .FIG .3

9 6 fi k z Emzma 530% 3 ZOFmDmEOO VOLTAGE DIFFERENCE ACROSS COMPARATOR COMBUSTION PRODUCT DETECTOR AND METHOD OF CALIBRATING This invention relates to combustion product detectors of the ionization chamber type and particularly to an improved detector capable of highly effective and reliable operation.

Many combustion product detectors utilizing ionization chambers have been provided heretofore and have proved useful for various applications and installations. For some detector systems extremely high reliability in operation and freedom from ambient air, temperature, pressure and moisture changes is essential as well as the elimination of false alarm indications such as those caused by transient conditions of the power supply. Accordingly, it is an object of this invention to provide a combustion product detector of the ionization chamber type including an improved arrangement for effecting accurate and reliable alarm operation.

It is another object of this invention to provide a combustion product detector including an improved arrangement for effecting accurate calibration of the alarm threshold of the detector.

It is another object of this invention to provide an improved combustion product detector of the ionization chamber type which is reliable and accurate in operation.

It is a further object of this invention to provide an improved method for calibrating combustion product detectors of the ionization chamber type.

Briefly, in carrying out the objects of this invention, in one embodiment thereof, solid state circuitry is employed in connection with a reference ionization chamber and a sampling chamber connected in series across a direct current supply at the output of a regulator. An insulated gate field effect transistor is connected across the supply with its gate connected to the junction between the chambers and produces an output signal corresponding to the density of combustion product in the sampling chamber. An integrated circuit comparator is provided to compare the output signal with an adjusted voltage representing the product density at which an alarm indication is required; when the voltage difference at the comparator input approaches zero an alarm signal is generated and energizes the alarm circuit. A reset circuit is provided which includes a normally saturated transistor in the comparator reference voltage circuit; resetting of the detector is effected in response to a drop of regulator input voltage below a predetermined value. The voltage across the comparator may be determined by connecting a voltmeter across isolation terminals provided for the purpose; this difference voltage is an indication of the product density setting and may be used for calibration of the detector.

In other embodiments of the invention periodic samplings of the comparator are provided by operation of an oscillator employing logic gates. Logic circuitry is also employed for utilizing the comparison signal for initiating operation of indicators or alarms.

The features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. The invention itself, however, both as to its organization and its manner of operation, together with further objects and advantages thereof, will best be understood by reference to the following description taken in conncction with the accompanying drawings in which:

FIG. 1 is a circuit diagram of one embodiment of the invention;

P10. 2 is a circuit diagram of a second embodiment of the invention;

FIG. 3 is a circuit diagram of a third embodiment of the invention; and

FIG. 4 is a set of curves indicating generally the type of operating characteristics existing at the comparators of the circuits.

In the embodiment of the invention illustrated in FIG. 1, a voltage regulator is connected across a DC power supply between a high side terminal 11 and ground. The output of the regulator 10 supplied to a line 12 is constant and two ionization chambers 13 and 14 are connected in series between the line 12 and ground. A transistor 15 is connected in series with a resistance 16 across the power supply in parallel to the ionization chambers 13 and 14. The transistor 15 is an N-channel depletion mode metal oxide silicon field effect transistor having its insulated gate electrode 17 connected to the junction between the chambers 13 and 14. The drain electrode 18 is connected to the high side of the power supply and the source electrode, indicated at 20, is connected to the resistor 16. The source electrode 20 and the substrate electrode 21 are connected together. The input current to the gate 17 is extremely low and prevents the chambers from loading while the high dynamic gain provides linear reproduction of the chamber signal. A capacitor 22 may, if desired, be provided between the gate 17 and ground to provide additional capacitance for slow alarm circuit response; this capacitance, however, will not be found necessary for the majority of applications. The signal from the transistor 15 is supplied to a comparator 23 through a resistance 24 which is connected in series with a capacitor 25 across the load resistance 16.

The comparator 23 is one half of an integrated circuit including a second half 26 which operates as a noninverting isolation amplifier between the transistor source 20 and a voltage test terminal 27.

The overall alarm response time is determined by the time constant of the circuit consisting of the resistor 24 and capacitor 25. The comparator 23 output indicated by the terminal or pin No. l switches from the low state to the high or alarm condition when the signal terminal from the source 20 becomes substantially the same as the potential at the pin or terminal 2 of the comparator 23. The terminal or pin 2 is connected to a resistance comprising portions 28, 30 and 31 connected in series to a control transistor 32 which is normally saturated so that the resistors are connected between the line 12 and ground. The pin numbers used herein are those of the integrated circuits employed. An adjustable contact 33 is set to provide a reference voltage substantially the same as and representing the potential of the output of the transistor 15 when there is a predetermined selected density of combustion product in the sampling chamber 14. When the difference in voltage between the pins 2 and 3 of the comparator 23 nears zero, the comparator latches by positive feedback through a diode 34 connected between the pins 1 and 3 and causes a blocking Zener diode 35 to conduct and turn on an alarm transmitter transistor 36 through a resistance 37, the base of the transistor 36 being connected to ground through a resistance 38; when the transistor 36 turns on it connects a light emitting diode 40 to the high side of the supply line 11 through a resistance 41 turning the diode on to indicate an alarm condition, and energizing an alarm relay winding 42 which is connected to the line 11 through a resistance 43. The resistance 43 limits the current flowing through the winding 42, and the winding is shunted by a diode 42 for inductive discharge suppression.

The second half of the integrated circuit, indicated at 26, is operated as a gain of one non-inverting isolation amplifier. When the sampling chamber 14 is quiescent, the voltage difference across the terminals 2 and 3 of the comparator 23 may be measured by a voltmeter connected between a terminal 27 and a second terminal 44 connected to the pin 2 of the comparator 23 and to ground through a capacitor 45. This provides a field measurement of the alarm threshold and further a voltmeter connected across the terminals 27 and 44 may be employed to change the calibration of the detector by adjusting the contact 33 of the resistance 30 while observing the voltage during the quiescent state of the transistor 15. This voltage difference is set to a value determined by test and which effectively indicates the threshold value at which the detector is set. Resistances 46 and 47 are isolation resistances for the terminals 27 and 44, respectively. Resistances 48 and 49 having their junction connected to pin 4 of the regulator are connected in series between the regulated voltage line 12 and ground and determine the value of the regulator output voltage.

A reset circuit is provided which includes the control transistor 32, resistances 51 and 52 and a Zener diode 53 which is connected between the resistance 52 and the supply line 11. Should the voltage at the high side of the supply line 11 fall below a predetermined value, the Zener diode ceases to conduct and the transistor 32 is rendered nonconducting or open. The voltage at which this occurs is selected as a voltage above the input voltage to the regulator 10 at which the desired output voltage can be maintained on the line 12. When the transistor 32 ceases to conduct the voltage drop across the resistors 28, 30 and 31 disappears and the terminal 2 of the comparator 23 goes high, the transistor 32 by shutting off thus prevents operation of the alarm. Power rectification of alternating current supplied to the supply line 11 is effected by capacitors 54 and 55 and a resistance 56 together with a diode 57 providing the required rectification, reverse voltage protection and line filtering.

A relay having a winding 58 shunted by an inductive discharge suppression diode 60 is connected between ground and the line 11 through a resistance 61. This relay is a supervisory relay and may be eliminated if desired. The circuit of the detector is not affected by the removal of the relay winding 58.

The embodiment of the invention illustrated in FIG. 2 includes components and circuitry essentially the same as that of FIG. 1 and corresponding parts have been designated by the same numerals with the suffix letter a". This second embodiment differs from that of the embodiment of FIG. 1 in that it employs signal sampling techniques and utilizes logic gates for the sampling and alarm control circuits.

Referring now to FIG. 2, the signal supplied from the field effect transistor 1511 through the resistance 24a across the capacitor 25a is sampled at selected periodic intervals by operation of an oscillator 65 which is connected to the No. 3 terminal of the comparator 23a through a resistance 66 and a diode 67 and samples the signal from the transistor 21a at intervals say once each second for say 40 to 80 milliseconds. This sampling arrangement provides high noise rejection except during the brief sampling period when the reference and signal voltages are compared. When the difference voltage between the pins 2 and 3 of the comparator 23a is substantially zero the comparator switches from its low state at the output pin 1 to a high state and this switches logic gates 68 and 70 to produce a high signal at the 1 and 2 pins of a gate 71 which is connected with a gate 72 as a flip-flop and latches the flip-flop pin 10 in its high or alarm condition. All of the gates 68, 70, 71 and 72 are NOR gates the outputs of which go high only when none of the input pins are high. The alarm signal is applied through a resistance 74 to the base of a transistor 75 which connects the alarm relay circuit to ground when it becomes conductive. Thus the relay winding 42a is energized and the light emitting diode 40a is also energized to indicate the alarm condition.

Upon operation of the alarm circuit when the output of the flip-flop 73 at pin 10 goes positive the operation of the sampling oscillator 65 is stopped and is locked in an alarm holding position at pin 4 of a three input NOR gate 77 and the oscillator will not again operate until the power to the detector is momentarily interrupted or reset. The sampling oscillator 65 includes two NOR gates 78 and 80, the output pin 3 of the gate 78 being connected to both inputs of the gate 80 which has its output connected to the diode 67 and to pin 5 of the gate 77. The feedback from the output of the gate 80 is provided through a capacitor 81 connected to the upper input pin of the gate 78 and to ground through a resistance 82. The timing of the oscillator is effected through a capacitance 83 and a resistance 84 connected in series between the high side of the regulator power at 12a and ground. For operation of the oscillator its two lower control pins 3 and 4 must be low. When the capacitance 83 charges above a predetermined voltage of about one half the potential of the supply 12a the lower pin of gate 78 connected to the junction of the capacitance 83 and resistance 84 causes the output at pin 3 to go low and the output at pin 4 of gate 80 to go high and capacitance 81 and resistance 82 provide a positive feedback loop causing regenerative feedback and one-shot operation for a brief period of, say, 40 to 80 milliseconds. When the output of gate 80 at pin 4 goes high the comparator 23 is sampled through diode 67. At the same time oscillator gate 77 goes low at its output pin 6 and discharges capacitor 83 through a diode 85 and a resistance 86, return to ground being effected through the ground connection in the integrated circuit including the gate 77. When capacitor 83 is discharged, the oscillator is ready for another cycle. The oscillator 65 having its threshold voltage at about one half supply voltage is highly noise immune and is thermally stable, furthermore, a change in the sampling rate of the oscillator will not affect the detector response or sensitivity.

In the event that the power supply voltage at the pin 11a drops because of surges or other transient conditions and the voltage at the input of the regulator 10a falls below a predetermined selected value higher than that at which the required voltage may be maintained on the line 12a the Zener diode 53a shuts off and transistor 32a becomes non-conducting. When transistor 32a is non-conducting its collector goes high and pin 3 of transistor 77 accordingly is high and the oscillator goes low and the flip-flop reverses setting off the alarm circuit through the transistor 75. Thus, in the event of such transients, the oscillator 65 and the alarm cannot be operated and a false alarm condition indication is avoided. The oscillator is then required to wait one full period after reset before the sampling operation can be resumed. As soon as the transient conditions on the line 11 disappear the Zener diode 53a again conducts and transistor 32a is returned to its normal saturated operating condition. Normal operation of the detector then continues.

The embodiment of this invention illustrated in FIG. 3 includes the same regulator and field effect transistor circuitry as that of FIGS. 1 and 2 and corresponding elements have been designated by the same numerals as those of FIG. 1 with the suffix letter b. The circuit of FIG. 3 also includes a sampling oscillator which is the same as that of FIG. 2 and corresponding elements have been designated by the same numerals as employed in FIG. 2 with the suffix letter b. A sensitivity adjustment has been provided at the input to the ionization chambers and comprises a voltage divider including resistances 88, 89 and 90. Resistance 88 is adjustable and acts as a potentiometer which on movement to the right from the position illustrated in a drawing raises the direct current bias on the ionization chamber 13b and 14b. The potentiometer 88 may be adjusted, by way of example, to vary the calibration within a density range of from 3 to 6 milligrams of combustion product in a cubic foot of air.

The output of the transistor 15b is supplied to a dual operational amplifier integrated circuit comprising ampliflers 92 and 93. Amplifier 93 is connected in the same manner as the amplifier 25 of FIG. 1, its pins 6 and 7 being grounded through a resistance 94 and being connected through an isolation resistance 95 to a test terminal 96.

The amplifier 92 which is the comparator has its No. 2 pin connected to the source of the field effect transis tor 15b and its No. 3 pin connected through a resistance 97 to the adjustable contact 33b on the resistance 30b. A capacitor 98 is connected between the contact 33b and ground and a calibration test terminal 100 corresponding to the terminal 44 of FIG. 1 is connected to the junction of the capacitor 98 and resistance 97, the resistance 97 acting as the isolation resistance for the terminal. The output signal from the transistor 15b is compared periodically with the reference signal from the voltage drop resistors 28b, 30b and 31b. The sampling being effected by operation of the oscillator 65b. Pin 3 of the comparator is normally near ground potential and is at the potential of the divider contact 33b only during the sampling period. Whenever the potential at pin 2 of the comparator 92 is lower than that at the pin 3, which is the condition when no ionized chamber product is present, then the oscillator sampling pulse is transmitted through a NOR gate 101 acting as an inverter. the output pin 11 of which goes low and allows a capacitor 102 to discharge through a diode 103 and a resistance 104 to the ground terminal (not shown) which is associated with the gate 101. The capacitor 102 is charged continuously through a resistance 106. The gate 101 and gates 78b, 80b and a gate 106 are parts of a single integrated circuit each of the gates being a two input NOR gate.

The positive going sampling pulse at the output of comparator 92 also turns on a light emitting diode through inverter 106, pin 10, transistor 107 and transistor 108; this produces a red flash once each second, the diode being connected to the supply through a resistance 110. A meter (not shown) may be connected between ground and a terminal 111 which is connected through a resistance 112 to the junction of the diode 105 and the collector of the transistor 108; this meter will indicate the negative going pulse while the diode is flashing.

When the ionization chamber 14b contains a sufficient density of combustion product to exceed the reference voltage at the comparator 92, the comparator output pin 1 will remain low and the light emitting diode will stop flashing. Under this condition the alarm capacitor 102 continues charging through resistance 106 and when the capacitance 102 reaches about one half the high side voltage a flip-flop 113 latches and pin 9 thereof goes high turning on a transistor driver 114 the base of which is connected to the point 9 by a resistance 115. This energizes a relay 116 through a resistance 117 and actuates the alarm. At the same time when the pin 9 of the flip-flop 113 goes positive the oscillator 65b stops and locks to an alarm holding position at pin 4 of gate 77!). The oscillator 65b operates in the same manner as the oscillator 65 of FIG. 2 and in addition is provided with a remote test terminal 118. The terminal 118 is connected across the resistance 84b of the oscillator by a resistance 120 and a diode 121. When the remote test is used the oscillator is stopped holding the output of the comparator 92 low. When the test line is held low for several seconds the alarm charging time constant determined by the capacitor 102 and resistance 106 reaches alarm threshold in the same manner as when combustion product is present in the sampling ionization chamber.

The alarm threshold status can be tested by a voltmeter connected between the test terminals 96 and 100 which make possible the field test measurement of the alarm threshold status. The transistor 122, resistances 123 and 124 and a Zener diode 125 operate as a reset network in a manner essentially similar to that of the transistor 32a and Zener diode 53a of FIG. 2, the Zener diode 125 shutting off below a predetermined voltage forcing the alarm latch pin 9 of the flip-flop 113 to the low state. During the reset the capacitor 102 is discharged so that false alarm indications will not be given. Pin 3 of gate 77b and pin 1 of the flip-flop 113 are held high. I

The pin 4 of gate 77b is connected to ground through a resistance 126 and a detachable jumper 127 may be provided in the connection between the resistance and the pin 9 of the-flip-flop 113. Removal of this jumper provides automatic resetting operation hence with the jumper removed the alarm latch pin 9 of the flip-flop 113 cannot place the oscillator in the hold position. Thus when the combustion product goes below the threshold value the oscillator will continue sampling and discharge the alarm integrator capacitor 102 the alarm latch is automatically reset upon restoration of transistor 122 to its conducting state by the resulting low state of the pin 1 of the flip-flop 113.

The voltage adjustment effected by the setting of the potentiometer 88 provides a sensitivity adjustment at the ionization chamber 14b, the DC bias being raised through the chambers by movement of the contact of the potentiometer 88 to the right. This also raises the bias at pin 2 of the comparator 92.

In the three detectors described above, the same transistors and the same integrated circuits may be employed. By way of example, the following components may be used.

Field effect transistors Type MU6l2 Other transistors 2N3569 Comparator and lsolation Amplifier Nsssxv Regulator NESSOA Referring now to FIG. 4 the three downwardly sloping lines 130, 131 and 132 represent characteristics for successively increased threshold densities of combustion product in the ionization chamber. These curves are plotted with combustion product density along the vertical axis and the voltage difference across the comparator along the horizontal axis. The three curves 130, 131 and 132 represent the operation of the comparator for threshold densities of 3, 6 and 9 milligrams per cubic foot, respectively. During the calibration of a circuit embodying the detecting and comparing features illustrated in the circuits of FIGS. 1, 2 and 3, calibration may be determined initially by testing the circuit under different combustion product densities in the air supplied to the sampling chambers 14, 14a and 14b. The difference voltages across the terminals 2 and 3 of the comparators are direct indications of the product density for which the detector has been set. Thereafter the tested devices and other devices of the same circuit component values and connections may be calibrated by selecting a voltage difference and setting the reference voltage to have a value greater than the quiescent output voltage of the field effect transistor by the voltage difference value taken from the curves. Thus a large number of detectors may be calibrated quickly so that each is calibrated for a predetermined threshold density of combustion product. Thus, an initial calibration of a plurality of like combustion product detectors quick and reliable calibration may be attained by the simple method of using a voltmeter to measure the voltage difference across the input terminals of the comparators during the quiescent condition of the sensing element of the detector. Furthermore, the provision of the readily available voltage test point makes the testing and checking ofa multiplicity of detectors provided in a system in a single building, by way of example, without having to run smoke or combustion product test on each detector in turn.

The resetting circuitry of this invention provides a simple and effective arrangement for preventing false alarm indications due to line surges or other transient conditions on the system so that normal operating condition of the detector is restored after the transient condition has disappeared and it is unnecessary to reset the system manually.

While this invention has been described in connection with specific embodiments of the circuitry thereof, various other modifications and applications will occur to those skilled in the art. Therefore, it is not desired that the present invention be limited to the details of circuitry and components illustrated and it is intended by the appended claims to cover all modifications which fall within the true spirit and scope of the invention.

I claim:

I. A combustion product detector of the ionization chamber type comprising:

a. a constant voltage direct current supply,

b. means including an air sampling ionization chamber and connected across said supply for producing an output signal proportional to the density of combustion product in the air of said chamber,

c. adjustable means including a resistance connected across said supply for providing a reference voltage representing a predetermined combustion product density in the chamber,

d. means for comparing said output signal and said reference voltage and for producing a second signal when said output signal and said reference voltage are substantially equal,

e. an alarm means, and

f. means responsive to said second signal for actuating said alarm means.

2. A combustion product detector of the ionization chamber type as set forth in claim 1 including readily accessible contact point means for affording measurement of the voltage difference between said reference voltage and said output signal whereby the alarm conditions for which said detector is set may be determined by a voltage reading during operation of said detector under normal conditions of the air.

3. A combustion product detector of the ionization chamber type as set forth in claim 2 wherein said contact point means comprises a first contact connected to the output of said signal producing means through a non-inverting isolation amplifier having a gain of one and a second contact connected to the reference voltage input of said comparing means.

4. A combustion product detector as set forth in claim 1 wherein said output signal producing means includes a reference ionization chamber connected across said supply in series with said first mentioned chamber and an insulated gate field effect transistor having its gate connected to the connection between said chambers and its output terminal connected to said comparing means.

5. A combustion product detector as set forth in claim 1 wherein said direct current supply includes a regulator and input means for connecting the regulator across a direct current source, and means for resetting said detector upon the occurrance of low voltage conditions at said input means, said resetting means including a normally saturated transistor connected in a circuit across said supply and having its base connected to the high side of said input in series with a normally conducting Zener diode, said diode becoming nonconducting at a low voltage higher than the minimum source voltage of said regulator and rendering said last mentioned transistor non-conducting.

6. A combustion product detector as set forth in claim 5 including an oscillator for activating said comparing means periodically to sample said signal, and wherein said resetting means includes means dependent upon the non-conducting condition of said last mentioned transistor for preventing operation of said oscillator and of said alarm means.

7. A combustion product detector of the ionization chamber type comprising:

a. a constant voltage direct current supply,

b. means including an air sampling ionization chamber and connected across said supply for producing an output signal proportional to the density of combustion product in the air of said chamber,

c. adjustable means including a resistance connected across said supply for providing a reference voltage representing a predetermined combustion product density in the chamber,

d. means for periodically comparing said output signal and said reference voltage and for producing a second signal when said output signal and said reference voltage are substantially equal, said comparing means comprising two two-input NOR gates connected in series, the output of the first gate being connected to both inputs of the second gate, a thru-input NOR gate connected in series with a diode and a second resistance and having one input terminal connected to the output terminal of said second gate and its output connected to said second resistance and said diode connected to one input of said first gate, a feedback capacitor connected between the output terminal of said second gate and the other input terminal of said first gate, and a feedback resistance connected between said other input and the common return of the supply, and a timing circuit comprising a resistance and a capacitance connected across the supply in series with their common terminals connected to said one input of said first gate,

. an alarm means, means responsive to said second signal for actuating said alarm means, and

g. means dependent upon actuation of said alarm means and connected to the other two inputs of said third gate for interrupting operation of said oscillator.

8. In the calibration of a plurality of like combustion product detectors of the ionization chamber type each including a signal sensing means for producing an output signal which increases in proportion to increases in density of combustion product, means for comparing the output signal with a reference voltage corresponding to the alarm density of combustion product and means for effecting an alarm upon increase of the value of the output voltage to that of the reference voltage, the method of calibrating the detector which comprises:

a. determining the quiescent output voltage of the sensing means at normal air conditions,

b. determining the output voltage of the sensing means at a plurality of increased values of combustion product density,

c. adjusting the reference voltage to the values of the output voltage at each of said plurality of values in turn,

(1. measuring the difference voltage across the comparing means for each of said plurality of values when the sensing means is in its quiescent state, and

e. setting each detector for alarm operation at a predetermined density of combustion products by adjusting the reference to obtain the voltage difference across the comparing means to each detector corresponding to the selected value of density. l 

1. A combustion product detector of the ionization chamber type comprising: a. a constant voltage direct current supply, b. means including an air sampling ionization chamber and connected across said supply for producing an output signal proportional to the density of combustion product in the air of said chamber, c. adjustable means including a resistance connected across said supply for providing a reference voltage representing a predetermined combustion product density in the chamber, d. means for comparing said output signal and said reference voltage and for producing a second signal when said output signal and said reference voltage are substantially equal, e. an alarm means, and f. means responsive to said second signal for actuating said alarm means.
 2. A combustion product detector of the ionization chamber type as set forth in claim 1 including readily accessible contact point means for affording measurement of the voltage difference between said reference voltage and said output signal whereby the alarm conditions for which said detector is set may be determined by a voltage reading during operation of said detector under normal conditions of the air.
 3. A combustion product detector of the ionization chamber type as set forth in claim 2 wherein said contact point means comprises a first contact connected to the output of said signal producing means through a non-inverting isolation amplifier having a gain of one and a second contact connected to the reference voltage input of said comparing means.
 4. A combustion product detector as set forth in claim 1 wherein said output signal producing means inclUdes a reference ionization chamber connected across said supply in series with said first mentioned chamber and an insulated gate field effect transistor having its gate connected to the connection between said chambers and its output terminal connected to said comparing means.
 5. A combustion product detector as set forth in claim 1 wherein said direct current supply includes a regulator and input means for connecting the regulator across a direct current source, and means for resetting said detector upon the occurrance of low voltage conditions at said input means, said resetting means including a normally saturated transistor connected in a circuit across said supply and having its base connected to the high side of said input in series with a normally conducting Zener diode, said diode becoming non-conducting at a low voltage higher than the minimum source voltage of said regulator and rendering said last mentioned transistor non-conducting.
 6. A combustion product detector as set forth in claim 5 including an oscillator for activating said comparing means periodically to sample said signal, and wherein said resetting means includes means dependent upon the non-conducting condition of said last mentioned transistor for preventing operation of said oscillator and of said alarm means.
 7. A combustion product detector of the ionization chamber type comprising: a. a constant voltage direct current supply, b. means including an air sampling ionization chamber and connected across said supply for producing an output signal proportional to the density of combustion product in the air of said chamber, c. adjustable means including a resistance connected across said supply for providing a reference voltage representing a predetermined combustion product density in the chamber, d. means for periodically comparing said output signal and said reference voltage and for producing a second signal when said output signal and said reference voltage are substantially equal, said comparing means comprising two two-input NOR gates connected in series, the output of the first gate being connected to both inputs of the second gate, a thru-input NOR gate connected in series with a diode and a second resistance and having one input terminal connected to the output terminal of said second gate and its output connected to said second resistance and said diode connected to one input of said first gate, a feedback capacitor connected between the output terminal of said second gate and the other input terminal of said first gate, and a feedback resistance connected between said other input and the common return of the supply, and a timing circuit comprising a resistance and a capacitance connected across the supply in series with their common terminals connected to said one input of said first gate, e. an alarm means, f. means responsive to said second signal for actuating said alarm means, and g. means dependent upon actuation of said alarm means and connected to the other two inputs of said third gate for interrupting operation of said oscillator.
 8. In the calibration of a plurality of like combustion product detectors of the ionization chamber type each including a signal sensing means for producing an output signal which increases in proportion to increases in density of combustion product, means for comparing the output signal with a reference voltage corresponding to the alarm density of combustion product and means for effecting an alarm upon increase of the value of the output voltage to that of the reference voltage, the method of calibrating the detector which comprises: a. determining the quiescent output voltage of the sensing means at normal air conditions, b. determining the output voltage of the sensing means at a plurality of increased values of combustion product density, c. adjusting the reference voltage to the values of the output voltage at each of said plurality of values in turn, d. measuring the differEnce voltage across the comparing means for each of said plurality of values when the sensing means is in its quiescent state, and e. setting each detector for alarm operation at a predetermined density of combustion products by adjusting the reference to obtain the voltage difference across the comparing means to each detector corresponding to the selected value of density. 